System and method for monitoring cardiorespiratory parameters

ABSTRACT

An apparatus, system, and method is disclosed for monitoring the motion, breathing, heart rate of humans in a convenient and low-cost fashion, and for deriving and displaying useful measurements of cardiorespiratory performance from the measured signals. The motion, breathing, and heart rate signals are obtained through a processing applied to a raw signal obtained in a non-contact fashion, typically using a radio-frequency sensor. Processing into separate cardiac and respiratory components is described. The heart rate can be determined by using either spectral or time-domain processing. The respiratory rate can be calculated using spectral analysis. Processing to derive the heart rate, respiratory sinus arrhythmia, or a ventilatory threshold parameter using the system is described. The sensor, processing, and display can be incorporated in a single device which can be worn or held close to the body while exercising (e.g., in a wristwatch or mobile phone configuration), or alternately placed in a fixed piece of exercise equipment at some distance form the body (e.g., in a treadmill dash panel), and may also be integrated with other sensors, such as position locators.

This application claims priority to U.S. Provisional Application Ser.No. 60/863,862 filed on Nov. 1, 2006, the entire contents of which areincorporated herein by reference. This disclosure relates to themonitoring of motion, breathing, and heart rate of living beings, e.g.,humans, in a convenient and low-cost fashion that is useful, forexample, in the assessment of cardiorespiratory markers of fitness andactivity, and more particularly to an apparatus, system, and method foracquiring, processing and displaying the corresponding information in aeasily understandable format. In this application, reference is made toa system which can measure motion, breathing and heart rate as acardiorespiratory monitoring device or system.

BACKGROUND

Monitoring of heart rate and respiration is of interest in assessing theperformance of cardiorespiratory systems. For example, measurements ofheart rate are useful when assessing fitness levels of humans, as thereare well-established guidelines for physiologically normal ranges ofheart rate in response to differing levels of activity. Measurements ofheart rate are widely used in fitness training schedules. For example,an exercise which maintains heart rate in a range between 100 and 120beats per minute (bpm) may be useful for fat-burning and endurancebuilding, whereas a competitive athlete may wish to undertake activitywhich raises the heart rate level to 160-180 bpm. Moreover, levels havebeen determined which reliably adjust for age and gender, so thatindividuals interested in a structured cardiovascular fitness programcan monitor their progress quantitatively. Accordingly, it is desirableto be able to measure heart rate in a variety of settings. However,reliable measurement of heart rate in exercise conditions poses certaintechnical challenges. While running or cycling, motion artifact cancorrupt heart rate measurements. While swimming, electrical measurementof heart rate can be difficult due to the conducting nature of water.

In addition to heart rate, breathing rate, depth and patterns are usefulindicators of the overall status of the cardiorespiratory system. It iswell observed that breathing rate increases in response to exercise, butthe rate of increase (or decrease during an exercise recovery period) isa marker of overall cardiorespiratory health. For persons withcompromised cardiorespiratory status, who might experience dyspnoea, forexample, the elevated respiratory rate is a useful marker of status.

Individual measurements of heart rate and respiration are of value, butin addition useful measurements can be derived from combinations ofthese measurements which provide overall markers. For example, it isknown that breathing directly modulates heart rate through aphysiological mechanism called respiratory sinus arrhythmia (RSA), inwhich the heart speeds up during inspiration, and decreases duringexpiration. RSA is particularly pronounced in young people, and tends todecline with age. However, in general, a high degree of RSA isassociated with health, and will change in response to exercise andchanges in diet (see for example, “Respiratory sinus arrhythmiaalteration following training in endurance athletes,” by Ronald E. DeMeersman, published in European Journal of Applied Physiology, vol. 64,no. 5, September 1992, pages 434-436). However, in order to quantifyRSA, simultaneous measurements of heart rate and respiration aredesirable.

Other useful parameters of cardiorespiratory fitness are the anaerobicthreshold (AT) and ventilatory threshold (VT). The anaerobic thresholdis the point at which the cardiorespiratory system is not providingsufficient oxygen to the muscles for the muscles' energy needs to befully met by aerobic metabolic processes. Accordingly, the body uses itsglycogen stores in an anaerobic metabolic process to maintain muscleoutput. At this point, the person has reached their maximum oxygenuptake, and will shortly become too fatigued to maintain their activitylevel (the maximum oxygen uptake is referred to as VO_(2,max)). Tomeasure AT accurately requires specialized laboratory equipment andblood sampling, so while this is used as a “gold standard”, it is notpractical for widespread use by individuals interested in fitness. Theventilatory threshold is related physiologically to the anaerobicthreshold. It is a point at which the response of minute ventilation(liters/min of air breathed) to exercise intensity becomes nonlinear,and is marked by a substantial increase in breathing rate. From anaerobic fitness point of view, it has been shown that the anaerobicthreshold and the ventilatory threshold are strongly correlated. Sincethe goal of many fitness programs is to increase AT, it is useful to beable to use VT as a reliable surrogate marker. The cardiorespiratorymonitor can be used to estimate VT by using combinations of respirationrate and heart rate. This will provide utility to the user of themonitor, as they can track the trends in their VT over long time periods(e.g., over the course of a fitness training program).

In the clinical setting, it is also useful to have reliable markers ofcardiovascular fitness. For example, people suffering from heart failurehave high exercise intolerance. Some subjects with heart failure arecandidates for heart transplant, but given the scarcity of availablehearts, doctors must prioritize patients in order of the severity oftheir disease. Again, for such cases, measurements of VT can be usefulin assessing the overall health of the patient. A discussion of thechallenges of assessing cardiorespiratory markers for assessing hearttransplantation candidates is given in D. Ramos-Barbón, D. Fitchett, W.J. Gibbons, D. A. Latter, and R. D. Levy, “Maximal Exercise Testing forthe Selection of Heart Transplantation Candidates—Limitation of PeakOxygen Consumption,” Chest. 1999; 115:410-417.

A large variety of techniques exist for measurement of heart rate forthe purposes of assessing cardiorespiratory fitness. Surface leadelectrocardiograms (ECGs) are a highly accurate way of capturing cardiacelectrical activity, and hence heart rate. However, they require thatthe subject attach gelled electrodes to the chest region, and also carryor wear the associated electronic processing and/or recording device. Sogenerally, full ECG measurement is restricted to clinical applications.

More convenient techniques for electrocardiogram measurement have beenintroduced which trade off signal quality for convenience, and are nowwidely used in commercially available heart rate fitness monitors. Thesetechniques use electrodes which are embedded in conductive textileswhich are placed in proximity to the skin. Typically, the textiles formpart of a chest band worn around the thorax at the level of the chest.Since the conductivity of the textile material is dependent on moisturecontent, these sensors work best when the person is exercisingvigorously and the skin is moistened with sweat (alternatively users canapply some conducting gel to ensure good electrical measurement). Thedisadvantage of this system is the requirement for the person to wearthe chest band, and the reduced signal quality when the person's skin isnot moist.

Another technique for assessing heart rate during exercise is to usepulse oximetry, which measures the changes in reflected/transmittedlight through blood vessels. A characteristic photoplethysmogram can begenerated in which each cardiac contraction is visible as a distinctpulse. However, pulse oximetry methods for measuring heart rate arelimited by motion artifacts and poor perfusion characteristics. Thepower requirements of the light emitting diodes used in oximeters canalso be a limiting factor in the battery life of such a device.

Respiratory effort and breathing rate can be also measured in multipleways. A common method for measuring respiratory effort uses inductanceplethysmography, in which a person wears a tightly fitting elastic bandaround their thorax, whose inductance changes as the person breathes inand out. A limitation of the method from a convenience point of view isthat the person has to wear a band, and remains connected to theassociated electronic recording device via wires. An alternative systemfor measuring respiratory effort is to use impedance pneumography, inwhich the impedance change of the thorax is measured. The limitation ofthis technology, is that it requires electrodes to be attached to thebody, and has an active electrical component which needs to be carriedby the subject.

For cardiorespiratory fitness assessment, it is also useful to measuregross bodily motion, as that is an overall indicator of daily activityand exercise intensity. The most common technique for measuringfree-living activity is to use accelerometers, which can measureacceleration. When carried by a person, such devices can provide auseful indicator of the overall duration and intensity of the person'smovement, such devices are often sold commercially as pedometers(step-counters). A limitation of this technology is the requirement forthe person to carry the device, and the limitations of the algorithmsfor converting measured acceleration into activity patterns.

What is needed then, is a method, system and apparatus for measuringheart rate, respiratory rate and effort, and motion, and which overcomesvarious limitations of conventional approaches.

SUMMARY

This disclosure provides various embodiments and aspects of anapparatus, system, and method for monitoring heart rate, breathing andmotion. In one embodiment, a sensor unit can be either worn (forambulatory use), or placed in a fixed position (e.g., as part of anexercise cycling machine). The sensor communicates with a processor anddisplay and, in one aspect, the sensor, processor, and display may bephysically implemented in the same unit. The processor may be used toextract information about heart rate, breathing and motion, and higherorder information (e.g., the current heart rate relative to previousepochs). The display is configured to provide feedback to the user, suchas displaying current heart rate or breathing rate. Feedback may also beprovided using sound (e.g., a beep for every heart beat detected). Inone aspect, a complete system may include one or more of a motion sensor(for detection of general bodily movement, respiration, and heart rate);a processing capability (to derive signals directly related to cardiacactivity, breathing and motion, and hence to derive parameters such asbreathing rate, heart rate, and movement); a display capability (toprovide visual feedback); an auditory capability (to provide acousticfeedback, e.g., a tone whose frequency varies with breathing, or a beepwith every detected heart beat); and/or a communications capability(wired or wireless) to transmit acquired data to a separate unit. Thisseparate unit may be configured to carry out the processing, display andauditory functions mentioned above.

In one or more embodiments, the disclosed system for measuring,analyzing, and displaying respiration, cardiac activity, and bodilymovement, comprises one or more sensors configured to receive areflected radio-frequency signal off a living subject, a processorconfigured to analyze the reflected signal to determine a measurement ofphysiological activity of the living subject; and a display arranged toprovide selected information relating to the physiological activity to auser of the system. The system may further comprise a transmitter thatgenerates the radio frequency signals that are reflected off the livingsubject, and the power levels emitted by the system are safe forcontinuous use with humans. The monitored physiological activitycorresponds to movements which can include breathing, cardiac activity,and large movements of the body (such as an arm swinging)

In another embodiment, a method for measuring, analyzing, and displayingrespiration, cardiac activity, and bodily movement includes receivingradio-frequency signals reflected from a human subject; analyzing thereflected signals to produce measurements relating to respiration,cardiac activity, and bodily movement of the human subject; andproviding selected information to a user of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described with reference tothe accompanying drawings in which:

FIG. 1 is a diagram illustrating a schematic of how a system of anembodiment might be used in assessment of exercise and activity; FIG. 1(a) shows an embodiment of the system as an upper arm cuff band; FIG. 1(b) shows the system as a clip-on device which can be attached to ashirt-pocket; FIG. 1( c) shows an example of the device worn as apendant around the neck; FIG. 1( d) illustrates the cardiorespiratorymonitor in a treadmill fitness system; FIG. 1( e) gives an example ofthe cardiorespiratory monitor embedded in an exercise cycle machine; andFIG. 1( f) shows the device as a wristwatch-like device while swimming.

FIG. 2 provides a schematic representation of a sensor element of oneembodiment.

FIG. 3 provides a representative raw sensor signal obtained when thesensor is close to the surface of the body (e.g., within 5 cm).

The upper curve of FIG. 4 shows the time course of aphotoplethysmographic signal obtained from an adult subject, where eachheart beat is associated with a distinctive pattern and the lower curveof FIG. 4 illustrates the signal obtained simultaneously from the samesubject at a distance of several meters, showing that there are separaterespiration and cardiac signals.

FIG. 5 illustrates the result of applying a technique for accessing andvisualizing the breathing and cardiac information using a time-frequencyrepresentation such as the short-time Fourier transform and apeak-finding algorithm.

FIG. 6 provides a schematic of the system when multiple radio frequency(RF) blocks similar to those depicted in FIG. 2 are used fortransmission and reception of the radio waves.

FIG. 7 illustrates a schematic of the display for the system.

FIG. 8 shows a schematic of how the system can calculate a parameterrelated to a ventilatory threshold.

DETAILED DESCRIPTION

FIG. 1 is a diagram illustrating a schematic of various environments inwhich the system might be used in assessment of exercise and activity.First, the device can be used in ambulatory applications (where theperson can move freely since they are wearing the cardiorespiratorymonitor). FIG. 1( a) shows an embodiment of the system as an upper armcuff band. FIG. 1( b) shows the system as a clip-on device which can beattached to a shirt-pocket. FIG. 1( c) shows an example of the deviceworn as a pendant around the neck, FIG. 1( d) illustrates thecardiorespiratory monitor in a treadmill fitness system, FIG. 1( e)gives an example of the cardiorespiratory monitor embedded in anexercise cycle machine and FIG. 1( f) shows the device as awristwatch-like device while swimming. The device can also be configuredfor use with other known exercise equipment.

FIG. 2 provides a schematic representation of an exemplary sensorelement. The sensor element uses radio-frequency sensing and processingto extract bodily motion associated with breathing and heart rate. Thebody motion associated with respiration is readily observable asbreathing induces motion of the thorax and abdomen. The motionassociated with cardiac activity is less obvious, but physiologists usethe term “ballistocardiogram” to refer to the pressure wave apparent atthe surface of the skin due to the cardiac contraction. This smallmotion can be detected by a sensitive motion sensor.

The system transmits a radio-frequency signal towards a person. Thereflected signal is then received, amplified and mixed with a portion ofthe original signal, and the output of this mixer is then low passfiltered. The output of this mixer can therefore be considered as aprocessed time-domain signal derived from the reflected radio-frequencysignal. This resulting signal contains information about the movement,respiration and cardiac activity of the person, and is referred to asthe raw sensor signal. In FIG. 2, the radio frequency sensor componentsof the system are illustrated with a pulsed continuous wave signal forillustration. In an alternative embodiment, the system may also usequadrature transmission in which two carrier signals 90 degrees out ofphase are used. In the limits that the pulse becomes very short in time,such a system can be recharacterized as an ultrawideband (UWB)radio-frequency sensor. Improved signal-to-noise ratio can also beobtained by using a continuous wave system, in which the RF signal iscontinuously transmitted.

FIG. 3 gives a representative raw sensor signal obtained when the sensoris close to the surface of the body (e.g., within 5 cm). The dominantcomponents in the received raw sensor signal will be theballistocardiogram, and the relative motion of the sensor and person. Toreduce the relative motion, the sensor unit may be mechanically fixed tothe skin using an elastic restraining mechanism, or similar. FIG. 3 isan example of the raw sensor signal with a dominant ballistocardiogramcomponent (in this case, measured at the inside of the elbow on theupper arm). This represents 5 seconds of data collected using a 26 GHzpulsed continuous wave prototype of the system. In such cases, heartbeats will be determined by a threshold passing technique (a pulse isassociated with the point where the signal is greater or less than thethreshold). In more complex (but typical cases), the ballistocardiogramwill present a more complicated but repeatable pulse shape. Therefore apulse shape template, implemented, for example, by a matched filter, canbe correlated with the acquired cardiac signal, and places where thecorrelation is high will be used as the heart beat locations.Accordingly the system recognizes cardiac beats of the living subject byidentifying peaks in the processed time-domain signal, or by carryingout a time-domain correlation of the received signal with a prototypicalcardiac signal, or by other means. This processing results in a seriesof time markers identifying the occurrence time of each heart beat.These time markers can be used by a processor to audibly signal eachheart beat of the living subject, or to light up an intermittent icon ona display.

Given a time marker of when each event occurred, calculating heart rateis possible. For the signal shown in FIG. 3, we will label the point atwhich the signal crosses a threshold as a cardiac event time B_(n)(where n is the beat number). From that we can calculate theinstantaneous heart rate as 1/BB_(n) where BB_(n)=B_(n)−B_(n-1) (theinterbeat interval). In practice, it may be more useful to define theaverage heart rate over a time epoch (e.g., 10 seconds). This can beachieved by counting the number of beats which occurred within a10-second window, and then dividing by 10 to obtain the average numberof beats per second. For the example shown in FIG. 3, 5.9 beats occurredwithin a five second window, so that the reported heart rate is(5.9/5)×60=71 beats per minute.

When the device is further away from the body (e.g., 1 meter or greater)the received raw sensor signal will be a combination of gross bodilymovement, respiration, and cardiac activity. The upper curve of FIG. 4shows the time course of a photoplethysmographic signal obtained from anadult subject, where each heart beat is associated with a distinctivepattern. The lower curve of FIG. 4 illustrates the signal obtainedsimultaneously from the same subject at a distance of several meters,and shows that there are separate respiration and cardiac signals.Specifically, the circles highlight the skin movement associated witheach cardiac beat. The skin motion is typically aligned with thedichrotic peak in the pulse waveform.

In cases of usage further away from the body, as described above thereceived raw signal contains information about breathing and heart rate,as well as gross bodily motion. A technique for accessing andvisualizing the breathing and cardiac information is to use atime-frequency representation such as the short-time Fourier transformand a peak-finding algorithm. The processor can also be configured torecognize the physiological activity of the living subject usingfrequency domain processing of the received signals. The detaileddescription of this is provided below, but broadly it consists of takingthe spectrum of an epoch centered at time t₁, and finding spectral peakswhich correspond best to the expected breathing and cardiac frequencies.For that epoch, the two peaks can be noted, and considered as thecardiac and respiratory frequency at time t₁. A new epoch can then beformed which overlaps with the previous epoch, but which is now centeredat t₂, and two new frequencies can be calculated which form the cardiacand respiratory frequency at time t₂. FIG. 5 illustrates the result ofapplying this technique to 50 seconds of data, with a window length of20 seconds, and an overlap of 19 seconds. The breathing component atabout 20 breaths per minute, and the cardiac component at approximately70 beats per minute can be tracked over time.

FIG. 6 provides a schematic of the system when multiple radio frequency(RF) blocks are used for transmission and reception of the radio waves.In this schematic, there are three independent RF blocks, each capableof receiving and transmitting the radio waves. The individual RF blocksare similar to that shown earlier in FIG. 2. They will generateindependent copies of the overall signal from the person being sensed,so that independent motion components can be extracted using signalprocessing (e.g., breathing, cardiac signal, and upper body motion).Note that the antennas can also transmit at separate frequencies ifrequired. Physical separation of the antennas (e.g., by greater than aquarter wavelength) will also make the transmission paths statisticallyindependent.

FIG. 7 illustrates a schematic of the display for the system. The systemwill typically display parameters such as current heart rate, currentbreathing rate, and the degree of respiratory sinus arrhythmia. Sincethe system may be easily integrated with a device capable of measuringposition (e.g., using the Global Positioning System—GPS), position mayalso be displayed on the system output. The system will also have thecapability to display useful trends for the user, such as the heart rateover the past hour, the values of RSA over the last week, etc. A furtheradvantage of incorporating position information is that it allows thesystem to be used in standard tests of fitness. For example, a goodmarker of general cardiovascular health is the “one mile fitness test”.In this, the person walks a mile briskly, and records their pulse at theend of the one-mile. A positioning system will automatically inform theperson when they have walked a mile, and record the heart rate at thattime. Similarly, in clinical applications, the six-minute walk test isroutinely used. In this, a person is asked to walk for six minutes attheir own pace, and the distance covered is a marker of their generalcardiovascular health. An integrated positioning system willautomatically keep a track of the distance covered, and the heart andrespiration rate during that period. So the utility of the system can beaugmented by including a positioning system configured to monitor alocation of the living subject, and to simultaneously track theirphysiological activity.

FIG. 8 shows a schematic of how the system can calculate a parameterrelated to ventilatory threshold. The device can record the heart rateand breathing rate over a period of exercise. At the end of theexercise, the device can plot heart rate versus the average breathingrate seen at that heart rate. A schematic representation of such a curveis shown in FIG. 8. If the exercise intensity is close to the person'smaximum, then the curve can be used to identify a “kink” at whichbreathing rate increases more rapidly with respect to heart rate. Thebreathing rate at which this occurs can act as a surrogate ofventilatory threshold (VT). The value of the this parameter can betracked over the course of weeks or months, as the person undergoes afitness program.

In one embodiment, the system includes a sensor unit, and a monitoringand display unit where results can be analysed, visualized andcommunicated to the user. The sensor unit and the display/monitoringunit can be incorporated into a single stand-alone device, if required.The device may include one or more of a motion sensor (for detection ofgeneral bodily movement, respiration, and heart rate); a processingcapability (to derive signals directly related to cardiac activity,breathing and motion, and hence to derive parameters such as breathingrate, heart rate, and movement); a display capability (to provide visualfeedback); an auditory capability (to provide acoustic feedback, e.g., atone whose frequency varies with breathing, or a beep with everydetected heart beat); a communications capability (wired or wireless) totransmit acquired data to a separate unit. This separate unit can carryout the processing, display and auditory capability mentioned above.

More specifically, the typical sensor will include one or moreradio-frequency Doppler sensors, which transmit radio-frequency energy(typically in the range of 100 MHz to 100 GHz), and which use thereflected received signal to construct a motion signal. For ease ofexplanation, we will first restrict our discussion to the case whereonly one sensor unit is used. The principle by which this works is thata radio-frequency wave

s(t)=u(t)cos(2πf _(c) t+θ)  (1)

is transmitted from the unit. In this example, the carrier frequency isf_(c), t is time, and θ is an arbitrary phase angle. u(t) is a pulseshape. In a continuous wave system, the value is always one, and can beomitted from Eq. (1). More generally, the pulse will be defined as

$\begin{matrix}{{u(t)} = \left\{ \begin{matrix}{1,{t \in \left\lbrack {{{kT}\mspace{14mu} {kT}} + T_{p}} \right\rbrack},{k \in Z}} \\0\end{matrix} \right.} & (2)\end{matrix}$

where T is the period width, and T_(p) is the pulse width. WhereT_(p)<<T, this becomes a pulsed continuous wave system. In the extremecase, as T_(p) becomes very short in time, the spectrum of the emittedsignal becomes very wide, and the system is referred to as anultrawideband (UWB) radar or impulse radar. Alternatively, the carrierfrequency of the RF transmitted signal can be varied (chirped) toproduce a so-called frequency modulated continuous wave (FMCW) system.

This radio frequency signal is generated in the sensor system using alocal oscillator coupled with circuitry for applying the pulse gating.In the FMCW case, a voltage controlled oscillator is used together witha voltage-frequency converter to produce the RF signal for transmission.The coupling of the RF signal to the air is accomplished using anantenna. The antenna can be omnidirectional (transmitting powermore-or-less equally in all directions) or directional (transmittingpower preferentially in certain directions). It can be advantageous touse a directional antenna in this system so that transmitted andreflected energy is primarily coming from one direction. The system iscompatible with various types of antenna such as simple dipole antennas,patch antennas, and helical antennas, and the choice of antenna can beinfluenced by factors such as the required directionality, size, shape,or cost. It should be noted that the system can be operated in a mannerwhich has been shown to be safe for human use. The system has beendemonstrated with a total system emitted average power of <1 mW (0 dBm)and lower. The recommended safety level for RF exposure is 1 mW/cm². Ata distance of 1 meter from a system transmitting at 0 dBm, theequivalent power density will be at least 100 times less than thisrecommended limit.

In all cases, the emitted signal will be reflected off objects thatreflect radio waves (such as the air-body interface), and some of thereflected signal will be received back at the transmitter. The receivedsignal and the transmitted signal can be multiplied together in astandard electronic device called a mixer (either in an analog ordigital fashion). For example, in the CW case, the mixed signal willequal

m(t)=γ cos(2πf _(c) t)cos(2πf _(c)+φ(t))  (3)

where φ(t) is the path difference of the transmitted and receivedsignals (in the case where the reflection is dominated by a singlereflective object), and γ is the attenuation experienced by thereflected signal. If the reflecting object is fixed, then φ(t) is fixed,and so is m(t). In the case of interest to us, the reflecting object(e.g., chest) is moving, and m(t) will be time-varying. As a simpleexample, if the chest is undergoing a sinusoidal motion due torespiration:

resp(t)=cos(2πf _(m) t)  (4)

then the mixed signal will contain a component at f_(m) (as well as acomponent centred at 2f_(c) which can be simply removed by filtering).The signal at the output of the low pass filter after mixing is referredto as the raw sensor signal, and contains information about motion,breathing and cardiac activity.

The amplitude of the raw sensor signal is affected by the mean pathdistance of the reflected signal, leading to detection nulls and peaksin the sensor (areas where the sensor is less or more sensitive). Thiseffect can be minimised by using quadrature techniques in which thetransmitter simultaneously transmits a signal 90 degrees out of phase(the two signals will be referred to as the I and Q components). Thiswill lead to two reflected signals, which can be mixed, leadingeventually to two raw sensor signals. The information from these twosignals can be combined by taking their modulus (or other techniques) toprovide a single output raw sensor signal.

In the UWB case, an alternative method of acquiring a raw sensor signalmay be preferred. In the UWB case, the path distance to the mostsignificant air-body interface can be determined by measuring the delaybetween the transmitted pulse and peak reflected signal. For example, ifthe pulse width is 1 ns, and the distance from the sensor to the body is0.05 m, then the total time m(τ) elapsed before a peak reflection of thepulse will be 0.1/(3×10⁸) s=0.33 ns. By transmitting large numbers ofpulses (e.g., a 1 ns pulse every 1 μs) and assuming that the pathdistance is changing slowly, we can derive a raw sensor signal as theaverage of the time delays over that period of time.

In this way, the radio-frequency sensor can acquire the motion of thepart of the body at which the system is aimed. Directional selectivitycan be achieved using directional antennas, or multiple RF transmitters.The combined motion of the thorax (which is a combination primarily of arespiration and cardiac signal) acquired in this way using a pulsedcontinuous wave system is shown in the lower curve of FIG. 4. We stresshowever that a continuous wave, an FMCW, or a UWB radar can also obtainsimilar signals.

Moreover, since the bulk of the reflected energy is received from thesurface layer of the skin, this motion sensor can also obtain theballistocardiogram, which is the manifestation of the beating of theheart at the surface of the skin due to changes in blood pressure witheach beat. An example of a surface ballistocardiogram obtained with anRF motion sensor has already been shown in FIG. 3. In that case, theballistocardiogram is emphasized by the sensor being close to the skin(upper arm) and no respiratory component is visible.

In order to improve the qualities of the measured sensor signals, thephysical volume from which reflected energy is collected by the sensorcan be restricted using various methods. For example, the transmissionantenna can be made “directional” (that is, it transmits more energy incertain directions), as can the receiver antenna. A technique called“time-domain gating” can be used to only measure reflected signals whicharise from signals at a certain physical distance form the sensor. Apractical way to implement this is to ensure that received signal ismixed with a transmitted signal over a predefined period of time. Forexample, imagine that a 12 ns pulse is emitted at time t=0 ns. If thereflecting object is 150 cm away, the reflected pulse will be firstreceived after 10 ns (since it takes light 10 ns to cover 300 cm).Assume a second object 300 cm away whose detection is not desired. Thereflected pulse from this second object will not first arrive till timet=20 ns. Therefore if mixing between the transmitted and received pulsesis only allowed in the time period from t=10 ns to t=15 ns, all theinformation received will relate only to the first reflecting object.Frequency domain gating can be used to restrict motions of the reflectedobject above a certain frequency.

In a simple embodiment of the system, a single antenna will be used,with a single carrier frequency. This antenna will act as both thetransmit and receive antenna. However, in principle, multiple receiveand transmit antennas can be used, as can multiple carrier frequencies.In the case of measurements at multiple frequencies (e.g., at 500 MHzand 5 GHz) the lower frequency can be used to determine large motionsaccurately without phase ambiguity, which can then be subtracted fromthe higher-frequency sensor signals (which are more suited to measuringsmall motion, such as the cardiac signature).

All of these sensor inputs are fed into the unit for processing anddisplay purposes, and for possible transmission to a separate unit (themonitoring unit).

The system then uses its processing capability to combine the sensorinputs to provide a number of useful outputs, and to display theseoutputs in a meaningful manner. These steps are carried out in thefollowing manner.

The cardiorespiratory monitor is primarily designed to provideinformation about heart rate and respiration. When the person is moving,the sensor signal will often be dominated by motion, in which caseprocessing is required to reduce motion artefact problems. A preferredtechnique for calculating respiration and heart beat activity in thepresence of noise is as follows.

A raw signal is acquired for an epoch of desired length (e.g., 20seconds). The spectrum of this period of the signal is estimated using atechnique such as the smoothed averaged periodogram. In general, sincerespiration occurs typically at a frequency from 10 to 25 breaths perminute (about 0.15-0.45 Hz), and cardiac activity occurs in the range60-120 beats per minute (1 to 2 Hz), the spectrum of the signal willhave two peaks in the ranges 0.15-0.4 Hz, and 1 to 2 Hz. The frequencyat which these peaks occur can be referred to as the breathing frequencyand the heart rate respectively, for that epoch. The results of thespectral analysis for each epoch can be arranged in time to form atime-frequency respiration plot, which is a useful means of visualizingthe overall respiratory and cardiac activity. Note that the epochs canoverlap, so that a breathing frequency and cardiac frequency can becalculated at arbitrary times (e.g., FIG. 5 shows the case where theanalyzed epochs are one second apart).

The presence of large motion artefacts may confound the processingdescribed above, so in some cases it may be necessary to preprocess thesignal to reduce the effect of motion artefact. Since large movementslead to large-magnitude signals in the processed time domain, aprocessor can be configured to measure the energy content of a filteredsignal, so that periods of bodily motion of the living subject arerecognized by comparing the energy content to a predetermined energyvalue. A method for doing this is to prefilter the epoch with a linearhigh pass filter (to remove all frequencies below 0.05 Hz, for example).An alternative would be to median filter the data with a window lengthof 10 seconds, and remove the median filtered signal from the originalsignal. Alternatively, we can recognise periods of motion by their highenergy content. These periods of motion may lead to artifacts in theprocessed signal, so suitable pectral analysis that removes periods ofmeasurement can be used. Specifically, when calculating the spectrum ofthe epoch, the data from these high motion sections is not included inthe estimation (using a technique called Lomb's periodogram whichprovides spectral estimates from data with missing segments).

An alternative processing technique for improving the accuracy of theheart beat and respiration detection is to acquire multiple signals frommultiple sensors. This is particularly beneficial in the case of highmotion artefact, such as the case when the system is used in a treadmillsetting with person jogging in the field of the sensors. In such a case,a preferred solution is to have multiple sensors (e.g, m, where m mighttypically be in the range four to sixteen, but can vary from one to anynumber). In practice (for cost reasons), it is probably efficient tohave a single transmit antenna, and multiple receive antennas only,rather than having each antenna be both transmitting and receiving.Likewise it may be beneficial to have the antenna or antennas generateRF signals at multiple frequencies. However, an embodiment of the methodis where one transmitter is used, and m signals are received in thesensor (each path will experience a different phase delay and amplitudechange). A further useful embodiment of the system is one in which thereare multiple sensors operating at different frequencies, wherein arelatively low frequency is used to estimate a large bodily movement ofthe living subject, and a relatively high frequency is used to estimatea smaller movement of the living subject. For example, a sensoroperating at 1 GHz would be useful for detecting movement in thecentimeter range, while a sensor operating in the same system at 100 GHzcould help detect movement of millimetres.

A useful model is to collect the m received signal into a vector ofsignals x:

$x = \begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{m}\end{bmatrix}$

It can be reasonable assumed that each signal represents a mixture ofreflections from multiple sources (e.g., one from breathing, one fromcardiac activity, one from left arm movement, etc.). Therefore, thereceived signals represent a linear mixture of sources w, so that

$w = {{{Ax}\mspace{14mu} {where}\mspace{14mu} A} = \begin{bmatrix}a_{11} & a_{12} & \ldots & \; \\a_{21} & \; & \; & \; \\\; & \; & \ddots & \; \\\; & \ldots & \; & a_{mm}\end{bmatrix}}$

In practice, we are interested in obtaining the signals w, since theywill cleanly separate the different components of interest. A criticalfactor which aids us in this analysis is that the source signals areindependent (i.e., the cardiac signal is independent of breathing, whichis independent of atm motion, for example). There are many algorithmswhich map the received x back to w, under this assumption, and these arereferred to as Independent Component Analysis (ICA) techniques. Inparticular, we can further optimise our solution by imposing certainconstraints on the source signals (e.g., it should have a dominantfrequency in the range 0.15 to 0.25 Hz). Such algorithms are calledconstrained ICA algorithms. A useful survey of techniques in ICAanalysis can be found in “independent component analysis for biomedicalsignals,” C. J. James and C. W. Hesse, Physiological Measurement vol. 26(1), R15-R39, February 2005.

As well as determining respiration rate and amplitude, cardiac rate, andmotion, the system provides for means to combine signals for calculationof further useful outputs. For example, a useful marker of overallcardiorespiratory health is respiratory sinus arrhythmia (RSA). Thismeasures the influence of breathing on heart rate, and the stronger thecoupling, the better the overall cardiorespiratory health. In general,there is utility in configuring a processor to calculate a parameter ofrespiratory sinus arrhythmia using the measured heart rate and breathingrate information. One approach may be to calculate a parameter ofrespiratory sinus arrhythmia using cross-spectral analysis of measuredheart rate and breathing rate signals.

However, a variety of techniques exists for calculating RSA. Oneembodiment for this system is as follows.

An epoch of measurement (e.g., 60 seconds) is taken, over which theperson's activity is fairly constant. The coherence between the cardiacsignal and the respiratory signal is obtained (coherence is typicallydefined as the ratio of the cross spectral density of two signalsdivided by the square root of the power spectral densities of thesignals taken separately.) The highest value of the coherence in adefined band (e.g., 0.15-0.25 Hz) is taken as a measure of the couplingbetween heart rate and respiration. This coherence value can be trackedacross different exercise sessions, or compared against a populationmean.

A further useful measure of cardiorespiratory performance obtained bythe system is the estimation of ventilatory threshold from heart ratemeasurements only, or combinations of heart rate and breathing rate. Thesystem can be configured to calculate useful parameters ofcardiorespiratory performance (such as ventilatory threshold) byrelating a measured heart rate to a measured breathing rate over adefined period of measurement. A preferred embodiment for capturingventilatory threshold from combined heart rate and breathing rate is toexamine a curve of cardiac beats per breathing cycle versus breathingrate. In such a curve, there is a characteristic kink, which occurs atthe frequency corresponding to the ventilatory threshold.

Finally, the system provides means for communicating useful informationto its user. The display means may be in a format such as a wristwatch,with parameters such as current heart rate, current breathing rate, andposition. The user may also have the ability to view trend screens,which show charts of previous heart rates over different time scales,previous breathing rates, as well as derived parameters such asestimated RSA coherence. In some use cases, it is beneficial to designan enclosure which can contain one or more sensors, the processor, andthe display. This enclosure could be suitable for being held in a handof the user for convenience of use. The enclosure could also incorporateother functionality such as telecommunications or positioning systems(e.g., a cellular phone handset would be a specific embodiment of suchan enclosure).

STATEMENT OF INDUSTRIAL APPLICABILITY

This disclosure has application in the medical, safety, and sportsfitness fields, for example, by monitoring motion, breathing, and heartrate of living beings, e.g., humans, in a convenient and low-costfashion. Such monitoring is useful, for example, in the assessment ofcardiorespiratory markers of fitness and activity of humans.

What is claimed is:
 1. A system for measuring, analyzing, and displayingrespiration, cardiac activity, and bodily movement, the systemcomprising: one or more sensors configured to receive a reflectedradio-frequency signal off a living subject; a processor configured toanalyze the reflected signal to determine a measurement of physiologicalactivity of the living subject; and a display arranged to provideselected information relating to the physiological activity to a user ofthe system.